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Full text of "Character and evolution of the ground-water flow system in the central part of the western San Joaquin Valley, California [microform]"



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CHARACTEETAND 

EVOLUTION OF THE 

GROUND-WATER FLOW 

SYSTEM IN THE 

CENTRAL PART 

OF THE WESTERN 

SAN JOAQUIN VALLEY, 

CALIFORNIA 






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U.S. GEOLOGICAL SURVEY 
Open-File Report 87-573 

REGIONAL AQUIFER SYSTEM ANALYSIS 



Prepared in cooperation with the 
SAN JOAQUIN VALLEY DRAINAGE PROGRAM 



This report was prepared by the U.S. Geological Survey in cooperation with 
the San Joaquin Valley Drainage Program and as part of the Regional Aquifer 
System Analysis Program of the U.S. Geological Survey. 

The San Joaquin Valley Drainage Program was established in mid-1984 and is j 
a cooperative effort of the U.S. Bureau of Reclamation, U.S. Fish and Wildlife 
Service, U.S. Geological Survey, California Department of Fish and Game, and* 
California Department of Water Resources. The purposes of the Program are to ' 
investigate the problems associated with the drainage of agricultural lands in 
the San Joaquin Valley and to develop solutions to those problems. Consistent 
with these purposes, program objectives address the following key areas: 
(1) Public health, (2) surface- and ground-water resources, (3) agricultural 
productivity, and (4) fish and wildlife resources. 

Inquiries concerning the San Joaquin Valley Drainage Program may be 
directed to: 

San Joaquin Valley Drainage Program 
Federal-State Interagency Study Team 
2800 Cottage Way, Room W-2143 
Sacramento, California 95825-1898 

The Regional Aquifer System Analysis (RASA) Program of the U.S. Geological 
Survey was started in 1978 following a congressional mandate to develop quanti- 
tative appraisals of the major ground-water systems of the United States. The 
RASA Program represents a systematic effort to study a number of the Nation's 
most important aquifer systems, which in aggregate underlie much of the country 
and which represent an important component of the Nation's total water supply. 
In general, the boundaries of these studies are identified by the hydrologic 
extent of each system, and accordingly transcend the political subdivisions to 
which investigations have often arbitrarily been limited in the past. The 
broad objective for each study is to assemble geologic, hydrologic, and 
geochemical information, to analyze and develop an understanding of the system, 
and to develop predictive capabilities that will contribute to an effective 
management of the system. The Central Valley RASA study, which focused on 
studying the hydrology and geochemistry of ground water in the Central Valley 
of California, began in 1979. Phase II of the Central Valley RASA began in 
1984 and is in progress. The focus during this second phase is on more 
detailed study of the hydrology and geochemistry of ground water in the San 
Joaquin Valley, which is the southern half of the Central Valley. 



CHARACTER AND EVOLUTION OF THE GROUND-WATER FLOW SYSTEM 
IN THE CENTRAL PART OF THE WESTERN SAN JOAQUIN VALLEY, 
CALIFORNIA 

By Kenneth Belitz 

U.S. GEOLOGICAL SURVEY 

Open-File Report 87-573 



REGIONAL AQUIFER SYSTEM ANALYSIS 



Prepared in cooperation with the 

SAN JOAQUIN VALLEY DRAINAGE PROGRAM 



m 
l 

M3 




Sacramento, California 
1988 



DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 



For additional information write to: 

District Chief 
U.S. Geological Survey 
Federal Building, Room W-2234 
2800 Cottage Way 
Sacramento, CA 95825 



Copies of this report 

may be purchased from: 

U.S. Geological Survey 

Books and Open-File Reports 

Box 25425 

Building 810, Federal Center 

Denver, CO 80225 



CONTENTS 



Page 

Abstract 1 

Introduction 2 

Purpose and scope 2 

Previous investigations 2 

Acknowledgments . 4 

Geology . 4 

Ground-water flow system 10 

Predevelopment flow system 10 

Agricultural development and system response 14 

Present-day configuration of the water table 28 

Vertical gradients 30 

Generalized geohydrologic section through the flow system 30 

Conclusions „ 32 

References cited 33 



ILLUSTRATIONS 

Page 
Figure 1-9. Maps showing: 

1 . Location and topography of the study area 3 

2. Thickness of Coast Range alluvium that overlies 

the Corcoran Clay Member of the Tulare Formation. 5 

3. Thickness and extent of Sierran sands..... 6 

4. Surficial geology 7 

5. Depth to the base of the Corcoran Clay Member 

of the Tulare Formation 8 

6. Thickness of the Corcoran Clay Member of the 

Tulare Formation 9 

7. Distribution of alkali in soils in western Fresno 

County . . . 11 

8. Boundaries of near-surface subsidence areas....... 12 

9. Estimated water-table altitude and extent of 

artesian areas, 1908 13 

10. Graph showing ground-water pumpage and total available water, 

Westlands Water District, 1935-85 14 

11-15. Maps showing: 

11. Areal extent of areas irrigated with ground 

water and surface water 15 

12. Potentiometric surface of the confined zone, 1952.... 16 

13. Water-table altitude, spring 1952 17 

14. Potentiometric surface of the confined zone, 

December 1967 18 

15. Land subsidence, 1926-72 19 



Contents III 



Figures 16-19. 



20. 
21. 



22. 
23. 
24. 



Page 
Maps showing: 

16. Potentiometric surface of the confined zone, 

spring 1984 20 

17. Depth to water table, spring 1952 21 

18. Depth to water table, October 1984 22 

19. Wells used to map the depth to and altitude of the 

water table , October 1984 23 

Hydrographs of two wells drilled to different depths in the 

semiconf ined zone 24 

Map showing location of area serviced by the regional tile- 
drain system and an area of approximately equivalent size 

and topographic and geomorphic location 25 

Hydrographs of eight wells along an approximate flow line 27 

Map showing water-table altitude, October 1984 28 

Generalized geohydrologic section through the flow system 31 



CONVERSION FACTORS 



The inch-pound system of units is used in this report. For those readers who 
prefer to use metric (International System) of units rather than inch-pound units, 
the conversion factors for the units used in this report are listed below. 



Multiply inch-pound unit 



By 



To obtain metric unit 



acres 

acre-feet per year 

feet 

feet per mile 

cubic feet per year per 

square foot 
miles 
square miles 



0.4047 
0.001233 

0.3048 
0.1894 
0.3048 



609 
590 



square hectometers 
cubic hectometers 

per year 
meters 

meter per kilometer 
cubic meter per annum 

per square meter 
kilometers 
square kilometers 



Sea level : In this report "sea level" refers to the National Geodetic Vertical 
Datum of 1929 (NGVD of 1929) — a geodetic datum derived from a general adjustment 
of the first-order level nets of both the United States and Canada, formerly 
called mean sea level of 1929. 



IV Contents 



CHARACTER AND EVOLUTION OF THE GROUND-WATER FLOW SYSTEM IN THE 
CENTRAL PART OF THE WESTERN SAN JOAQUIN VALLEY, CALIFORNIA 



By Kenneth Belitz 



ABSTRACT 

The occurrence of selenium in agricul- 
tural drain water derived from the west- 
ern San Joaquin Valley, California, has 
focused concern on the ground-water flow 
system of the western valley. In this 
investigation, previous work and recently 
collected texture and water-level data 
are used to evaluate the character and 
evolution of the regional ground-water 
flow system in the central part of the 
western valley, with particular emphasis 
on the deposits overlying the Corcoran 
Clay Member of the Tulare Formation. 

The Corcoran Clay Member, where pre- 
sent, divides the flow system into an 
upper semiconfined zone and a lower con- 
fined zone. Above the Corcoran, three 
hydrogeologic units can be recognized: 
Coast Range alluvium, Sierran sand, and 
flood-basin deposits. These units differ 
in texture, hydrologic properties, and 
oxidation state. 



The development of irrigated agricul- 
ture in the central part of the western 
valley has significantly altered the flow 
system. Percolation of irrigation water 
past crop roots has caused a rise in the 
altitude of the water table in midfan and 
distal-fan areas. Pumpage of ground 
water from wells has caused a lowering of 
the water table beneath parts of the 
fanheads and a lowering of the potentio- 
metric surface of the confined zone over 
much of the western valley. The combi- 
nation of percolation and pumpage has 
resulted in development of a large down- 
ward hydraulic head gradient in the semi- 
confined zone and has created a ground- 
water divide along the western margin 
of the valley. Surface-water deliveries 
from the California Aqueduct have allowed 
a decrease in pumpage and a consequent 
recovery in hydraulic head throughout 
the system. 



Abstract 



INTRODUCTION 

Saline conditions and associated high 
levels of selenium and other soluble 
trace elements are prevalent in soils, 
ground water, and agricultural drain 
water of the western San Joaquin Valley, 
California (Deverel and others, 1984; 
Tidball and others, 1986) . The occur- 
rence and movement of selenium and other 
dissolved constituents through the hydro- 
logic system of the western valley is 
closely related to the movement of ground 
water. Therefore, an understanding of 
the ground-water flow system should help 
provide insight into the sources, occur- 
rence, and movement of selenium and other 
solutes in the system. In addition, an 
increased understanding of the ground- 
water flow system will provide resource 
managers with information that will be 
helpful in managing the system. 



Purpose and Scope 

The objective of this report is to 
present an overview of the ground-water 
flow system in the central part of the 
western San Joaquin Valley, with particu- 
lar emphasis on the deposits and flow 
system that overlie the Corcoran Clay 
Member of the Tulare Formation. The 
primary study area is shown in figure 1 , 
though some information given in this 
report extends beyond those boundaries. 

The study area includes those parts of 
the western valley containing the highest 
levels of selenium in soil (Tidball and 
others, 1986) , ground water, and agri- 
cultural drain water (Deverel and others, 
1984) . Although there have been several 
reports on the ground-water hydrology 
of the western valley, few have focused 
on the flow system in the deposits that 
overlie the Corcoran. Moreover, the 
flow system has undergone considerable 
change since those reports have been 
written. The present investigation 
synthesizes previous work with ongoing 
investigations to describe (1) the 
geology of the regional flow system, 
(2) the evolution of the flow system 



since the development of irrigated agri- 
culture, and (3) the present day flow 
system. Such a synthesis will provide 
valuable information to current investi- 
gators and planners and will serve as a 
foundation for subsequent quantitative \ 
studies of the flow system at both local 
and regional scales. 

This study is part of a comprehensive 
investigation by the U.S. Geological 
Survey of the hydrology and geochemistry 
of the San Joaquin Valley. The studies are | 
being done as part of the Regional Aquifer j 
System Analysis Program of the U.S. , 
Geological Survey and in cooperation with 
the San Joaquin Valley Drainage Program. 



Previous Investigations 

Several geologic and hydrologic studies 
have focused on or included the central 
part of the western San Joaquin Valley. 
Davis and Poland (1957) recognized three 
bodies of ground water in the western San 
Joaquin Valley: (1) an unconfined" and 
semiconfined zone of fresh water above the 
Corcoran Clay Member of the Tulare Forma- 
tion, (2) a confined zone of fresh water 
beneath the Corcoran Clay Member, and (3) 
a saline body of water underlying the con- 
fined fresh water. Davis and Poland (1957) 
and Davis and others (1959) noted that the 
deposits overlying the Corcoran Clay 
Member are derived from the Coast Range to 
the west and the Sierra Nevada to the 
east. Miller and others (1971) mapped the 
thickness and extent of the deposits 
derived from each of these sources. Miller 
and others (1971) and Bull and Miller 
(1975) noted that the deposits derived 
from the Coast Range that overlie the 
Corcoran are typically of low permeability 
and those derived from the Sierra Nevada 
are generally of higher permeability. 
Historically, agricultural wells in the 
western valley primarily have tapped the 
confined zone except in the valley trough, 
where the wells also tap the Sierran 
sands. The geology of the freshwater 
bearing deposits is extensively discussed 
by Miller and others (1971) , Croft (1972) , 
Hotchkiss (1972) , and Page (1986) . 



2 Ground-Water Flow System, San Joaquin Valley, California 



121°00 



120°30' 



120°00' 



37°00' 



36°30' 



36°00' 




T" 

10 20 KILOMETERS 

CONTOUR INTERVAL 40 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 

FIGURE 1. - Location and topography of the study area. 



Several of the previous studies have 
assessed the ground-water flow system at 
a particular period in time. The earli- 
est assessment of ground water in the 
western valley was by Mendenhall (1908) 
and by Mendenhall and others (1916) . 
Their work provides documentation of the 
system during the earliest stages of 
agricultural development. Davis and 
Poland (1957) and Davis and others (1959) 



provide documentation of the flow system 
during a period of rapid expansion of 
irrigated acreage. These authors used 
the work of Mendenhall and others (1916) 
to assess the natural flow system and 
evaluate the changes in the natural 
system resulting from agricultural 
development. Bull and Miller (1975) also 
investigated changes in the flow system 
arising from agricultural development and 



Introduction 



related those changes to land subsidence. 
The U.S. Bureau of Reclamation (1965) 
prepared maps, cross sections, and 
hydrographs, which document the geology 
and hydrology prior to the completion of 
the California Aqueduct. Hotchkiss and 
Balding (1971) assessed the geology, 
hydrology, and water quality of the 
freshwater bearing deposits in the 
northern part of the western valley. 
Ireland and others (1984) present a large 
number of maps and hydrographs that 
document water levels and the evolution 
of the flow system. 

Most recently, Williamson (1982), 
Diamond and Williamson (1983) , and 
Williamson and others (1985) conducted a 
comprehensive investigation of the flow 
system for the entire Central Valley. 
That study, which was part of the 
Regional Aquifer System Analysis (RASA) 
program of the U.S. Geological Survey, 
provides an assessment of the flow system 
in the Central Valley under natural 
conditions and for 1961-77. 



Acknowledgments 

The preparation and completion of this 
report were made possible by the coopera- 
tion and assistance of several agencies 
and individuals. The U.S. Bureau of 
Reclamation, the California Department of 
Water Resources, and the Westlands Water 
District provided data and maps essential 
to this investigation. Steven Phillips 
and Jo Ann Murashige, of the U.S. 
Geological Survey, assisted the author in 
the reduction of data and preparation of 
maps. The author would especially like to 
thank Frederick Heimes and John Carlson 
of the Survey for their production of the 
maps presented in this report. 



GEOLOGY 

The San Joaquin Valley is an asymmet- 
rical basin bounded by the Coast Ranges 
to the west, the Tehachapi Mountains to 
the south, the Sierra Nevada to the east, 
and the delta of the San Joaquin and 



Sacramento Rivers to the north. The axis 
of the valley trough is closer to the 
Coast Ranges than to the Sierra Nevada. 
The study area shown in figure 1 is 
defined by the Coast Ranges to the west, 
by the San Joaquin River and Fresno 
Slough in the trough of the San Joaquin 
Valley to the east and, to the north and 
south, by the lateral extent of prominent 
alluvial fans derived from the Coast 
Ranges . 

Bull (1964a, 1964b, 1972) identified 
21 alluvial fans in the central part of 
western Fresno County ranging in area 
from less than 1 square mile to more than 
250 square miles. Deposits associated 
with the two largest fans, Los Gatos 
Creek fan and Panoche Creek fan, occupy 
more than one-half the total area. Many 
of the fans identified by Bull (1964a 
1964b, 1972) are of limited areal extent 
and coalesce with large neighboring fans. 

The Pleistocene Corcoran Clay Member of 
the Tulare Formation divides the ground- 
water flow system into a lower confined 
zone and an upper semiconfined zone. The 
deposits of the semiconfined zone can be 
divided into three hydrogeologic units: 
Coast Range alluvium, Sierran sands, and 
flood-basin deposits. These units differ 
in texture, hydrologic properties, and 
oxidation state. 



The Coast Range alluvium is derived 
from the Coast Range to the west. The 
alluvial deposits are generally oxidized 
(Davis and others, 1959) and range in 
thickness from 850 feet (Page, 1986) 
along the Coast Range to feet along the 
valley trough (fig. 2). The texture of 
the alluvium is largely a function of 
relative position on the alluvial fan. 
Alluvial fans are commonly divided into 
three parts (Blissenbach, 1954; Reineck 
and Singh, 1980) : the apex of the 
alluvial fan (the fanhead) , the area 
between the fanhead and the lower margins 
of the fan (the midfan) , and the outer- 
most, lowest altitudes of the fan, where 
the fan often coalesces with other fans 
(distal fan) . The deposits are signifi- 
cantly coarser at the fanheads than at 



Ground-Water Flow System, San Joaquin Valley, Calif ornic 




120°30' 



120°00' 



37°00' 



•o >' Los Banus 



^ SAN LUIS , <2> ,' ; 
fRESERVOIH % a 



36°30 



EXPLANATION 

—1 oo-- LINE OF EQUAL THICKNESS OF 

COAST RANGE ALLUVIUM - Dashed 
where approximately located. Interval, 
100 feet 

BOUNDARY OF CORCORAN CLAY 

MEMBER OF THE TULARE 
FORMATION 

BOUNDARY OF VALLEY DEPOSITS 



36°00' 




"<%*€«** ^%S 



- 1 - 
10 



10 

I 



20 MILES 



- 1 

20 KILOMETERS 



FIGURE 2. - Thickness of Coast Range alluvium that overlies the Corcoran Clay Member of the Tulare Formation. 

(Adapted from Miller and others, 1971.) 



midfan and distal-fan locations. Text- 
ural analysis of the alluvial deposits 
(J. A. Laudon, U.S. Geological Survey, 
written commun., 1987) indicates that 
the fanhead deposits are typically 
80 to 100 percent sand and gravel and 
less than 20 percent silt and clay. 
The distal-fan deposits typically 
contain less than 20 percent sand 
and gravel and more than 80 percent 
silt and clay. The midfan deposits 
are typically coarse textured along 



present-day stream channels and paleo- 
channels and finer grained between 
channels. 

Bull (1964b) recognized two types of de- 
posits in the alluvium: mudflow and water- 
laid deposits. The mudflow deposits typi- 
cally are poorly sorted and are in close 
proximity to the Coast Range. The water- 
laid deposits are better sorted and are 
more areally extensive than the mudflow 
deposits . 



Geology 



The alluvium derived from the Coast 
Range interfingers eastward with material 
derived from the Sierra Nevada to the 
east. In the trough of the valley, the 
deposits derived from the Sierra Nevada 
are predominantly well-sorted micaceous 
sands (Miller and others, 1971). The 
Sierran sands are 400 to 500 feet thick 
in the valley trough and thin eastward 



and westward (fig. 3) . The Sierran sands 
differ from the Coast Range alluvium in 
texture as well as oxidation state. In 
contrast to Coast Range alluvium, the 
Sierran sands are reduced in the valley 
trough (Davis and others, 1959) . The 
Sierran deposits are highly permeable and 
historically have been tapped by wells as 
a source of irrigation water. 



121°00' 



120°30' 



120°00' 



37°00 



36°30 



36°00 




10 20 KILOMETERS 

FIGURE 3. - Thickness and extent of Sierran sands. (Adapted from Miller and others, 1971.) 



6 Ground-Water Flow System, San Joaquin Valley, Calif orni; 



In the valley trough, the Sierran sands 
are overlain by a veneer of flood-basin 
deposits (fig. 4) . The basin deposits 
are derived from the Coast Range to the 
west and the Sierra Nevada to the east. 
The deposits are of variable thickness 
(typically 5 to 35 feet) and texture 
but consist primarily of fine-textured, 



moderate to densely compacted clays. The 
basin clays are of low permeability and 
greatly impede the downward movement of 
water. The oxidation state of the flood- 
basin deposits is variable, reflecting the 
variability of depositional conditions 
and perhaps changes in oxidation state 
attributable to agricultural activity. 



121°00' 



120°30' 



120°00' 



37°00' 



36°30 




•3° ' \% 

K /-•■ 



*\**+ * * 

* ij+^f + + + 
+ K -Co + + + 

* * ^ + + + 
..rSnqitilit^A* + ♦ +i 
+ + + +\ ■8amioanuin + 

t •♦ + + + 
♦ + + + 



M 



EXPLANATION 

ALLUVIAL DEPOSITS 
STREAM CHANNEL DEPOSITS 
FLOOD-BASIN DEPOSITS 
PLEISTOCENE NONMARINE DEPOSITS 
OTHER NONMARINE DEPOSITS 

CONTACT 



Cantua^ 'L . 



'"\* 






<^y c 






36° 00' 



_ 



■; /, BOUNDARY OF VALLEY DEPOSITS 




■ 



'%> 'V-- 



m 



10 



20 MILES 

_l 



I I 1 

10 20 KILOMETERS 

FIGURE 4. - Surficial geology. (Adapted from California Division of Mines and Geology, 1959, 1965, and 1966.) 



Geology 



An understanding of the flow system 
above the Corcoran Clay Member of the 
Tulare Formation requires, to a certain 
extent, an understanding of the Corcoran 
and of the flow system below the 
Corcoran. The Corcoran Clay Member is 
an extensive lacustrine deposit of low 
permeability (Johnson and others, 1968) . 
The base of the unit ranges in depth 
from 400 feet in the valley trough to 
more than 900 feet along the Coast Range 



(fig. 5) and the unit ranges in thickness 
from 20 to 120 feet (fig. 6). Recent 
mapping of the Corcoran Clay Member 
(Page, 1986) indicates that the structure 
and thickness of the Corcoran is more 
variable than indicated in figures 5 
and 6. The general trends, however, 
illustrated in figures 5 and 6 are con- 
firmed by the work of Page (1986) . The 
upper two-thirds of the Corcoran Clay 
Member consists of thin-bedded clayey 



121°00' 



120"30' 



120°00' 



37°00 



36°30 



36°00' 




10 20 KILOMETERS 

FIGURE 5. - Depth to the base of the Corcoran Clay Member of the Tulare Formation. (Adapted from Bull and Miller, 1975.) 



8 Ground-Water Flow System, San Joaquin Valley, California 



121*00' 



120°30' 



120°00' 



37°00' 



36°30 



36°00' 




10 20 KILOMETERS 

FIGURE 6. - Thickness of the Corcoran Clay Member of the Tulare Formation. (Adapted from Miller and others, 1971.) 



silt and silty clay and the lower one- 
third consists of interbedded sand-silt- 
clay and clayey silt (Bull, 1975). The 
Corcoran is chemically reduced except in 
the extreme western part of the study 
area where it has been uplifted and 
partially oxidized. 

Historically, many of the agricultural 
production wells in the study area have 



been perforated below the Corcoran Clay 
Member. The lower confined zone con- 
sists of poorly consolidated flood-plain, 
deltaic, alluvial-fan, and lacustrine 
deposits of the Tulare Formation. The 
lower confined zone has been the sub- 
ject of numerous investigations, many 
of which focused on land subsidence 
(for a review, see Poland and others, 
1975) . 



Geology 



GROUND-WATER FLOW SYSTEM 

The ground-water flow system of the 
western San Joaquin Valley has undergone 
considerable change since the development 
of irrigated agriculture. The present- 
day flow system is in a transient state 
and is responding to stresses imposed on 
it in both the past and the present. 
Therefore, it is useful to understand the 
natural flow system and the evolution of 
the flow system since the development of 
irrigated agriculture in the western val- 
ley. Because the focus of this paper is 
on the semiconfined zone, it is of par- 
ticular importance to understand those 
activities that have affected the semi- 
confined zone. These activities include 
percolation of irrigation water past crop 
roots, historic pumpage of ground water 
from below the Corcoran Clay Member of 
the Tulare Formation, delivery of surface 
water, and installation of a regional 
subsurface tile-drain system. As will be 
discussed in the following sections, the 
response of the semiconfined zone to 
these activities is partly dependent on 
the texture of the subsurface deposits. 



Predevelopment Flow System 

Under natural conditions, recharge was 
primarily from infiltration of stream 
water from intermittent streams and, per- 
haps, from smaller ephemeral streams. 
The intermittent streams (Little Panoche, 
Panoche, Cantua, and Los Gatos Creeks) 
flow seasonally during the winter rainy 
season and the smaller ephemeral streams 
flow only after storms (Bull, 1964a) . 
None of the stream courses reach the San 
Joaquin River or Fresno Slough in the 
trough of the valley. The streams lose 
their flow through evaporation and infil- 
tration before reaching the valley floor. 
Davis and Poland (1957) estimated that 
the four intermittent creeks typically 
have a total flow of about 50,000 acre- 
feet per year (acre-ft/yr) , of which 
30,000 to 40,000 acre-ft/yr infiltrates 
and recharges the ground water. Davis 
and Poland (1957) and subsequent workers 



assumed that rainfall was an insignifi- 
cant mechanism for recharging the system 
under natural conditions. 

Soil-salinity and soil-compaction data 
support the view that recharge was limit- 
ed primarily to areas traversed by the 
intermittent streams. A soil-salinity 
map prepared by Harradine (1950; fig. 7) 
indicates that the highest soil salini- 
ties are associated with the midfan and 
distal-fan, areas which historically have 
not been traversed by intermittent 
streams. The lowest soil salinities are 
associated with the fanhead areas and 
with the midfan areas, where they are 
traversed by the major creeks. The 
absence of saline soils along the contact 
between the Coast Range and the alluvium 
along much of the western valley suggests 
that the numerous minor drainages may 
have contributed recharge to the ground- 
water flow system in the geologic past. 
In contrast, the distribution of soils 
that have been or would likely be 
susceptible to near-surface subsidence 
resulting from soil compaction caused 
by application of water at the surface 
(fig. 8) indicates that recharge was 
small in the areas between the large 
alluvial fans of the intermittent creeks. 
Bull (1964b) noted that the interfan 
areas were built up from mudflow 
deposits, which are subject to compaction 
when wetted. The presence of deposits 
compacted prior to agricultural develop- 
ment in the fanhead areas and of deposits 
that have been compacted as a result of 
irrigation (or would likely be compacted 
if wetted) in the interfan areas suggests 
that recharge occurred under natural 
conditions at the fanheads and did not 
occur in the interfan areas. 

Discharge from the system under natural 
conditions was primarily by evapotrans- 
piration and streamflow along the valley 
trough. Early geologic surveys of the 
valley (Hamilton, 1916) indicate the 
presence of marshland along most of the 
valley trough. Mendenhall and others' 
(1916) map of water levels in the semi- 
confined zone (fig. 9) indicates that 



10 



Ground-Water Flow System, San Joaquin Valley, Califo 



rnia 



121°00' 



120°30' 



120°00' 



37°00 




36°30 



10 20 KILOMETERS 

FIGURE 7. - Distribution of alkali in soils in western Fresno County. (Adapted from Harradine, 1950.) 



artesian conditions prevailed along a 
broad stretch of the valley trough. The 
presence of marshlands in the arid to 
semiarid Central Valley and the extensive 
artesian conditions indicate that the 
valley trough was a discharge area under 
predevelopment conditions. 

Mendenhall's map is based on water 
levels in wells perforated in the semi- 
confined zone. His map indicates that 



ground-water gradients in the semi- 
confined zone were from the southwest to 
the northeast, reflecting the general 
topographic trend of the area. Gradients 
typically were 1 to 3 feet per mile, re- 
flecting the arid climate and low rates 
of recharge to the system. Mathematical 
simulation of the ground-water flow 
system (Williamson and others, 1985) 
indicates that the hydraulic head distri- 
bution in the confined zone beneath the 



Ground-Water Flow System 11 



120°30' 



120°00' 



37°00' 



36°30 



36°00' 




EXPLANATION 

AREAS OF KNOWN SUBSIDENCE 

AREAS OF PROBABLE 
SUBSIDENCE, IF IRRIGATED 

AREAS OF POSSIBLE 
SUBSIDENCE, IF IRRIGATED 

BOUNDARY OF VALLEY DEPOSITS 



Kettleman 
City 



FIGURE ' 



10 20 KILOMETERS 

— Boundaries of near-surface subsidence areas. (Adapted from Bull. 1964b.) 



12 Ground-Water Flow System, San Joaquin Valley, California 



120°30 



120°00' 



37°00' - 



36°30' 



36°00' 




EXPLANATION 

-740— WATER-TABLE CONTOUR - Shows 
altitude of water table. Contour 
interval, 20 feet. Datum is sea level 



ARTESIAN AREA 



~ BOUNDARY OF VALLEY DEPOSITS 



> 






] 



10 

_L_ 



20 MILES 
_| 



10 20 KILOMETERS 

FIGURE 9. — Estimated water-table altitude and extent of artesian areas, 1908. (Adapted from Mendenhall and others, 1916.) 



Ground-Water Flow System 13 



Corcoran Clay Member of the Tulare 
Formation was quite similar to the 
hydraulic head distribution in the upper 
semiconf ined zone. The numerical simula- 
tion model of Williamson and others 
(1985)' indicates that hydraulic head in 
the lower confined zone was typically 10 
to 20 feet lower then the hydraulic head 
in the upper semiconfined zone along the 
Coast Range and to 10 feet higher along 
the valley trough. 

Agricultural Development and 
System Response 

Agricultural activity in the study area 
began as early as the 1870' s, but large- 
scale farming and irrigation did not 
occur until the First World War (Davis 
and Poland, 1957; Bull and Miller, 1975). 
Irrigation with ground water expanded 
rapidly in the 1920' s and steadily 
increased until World War II. After 
World War II, the price of commodities 



stimulated increased agricultural growth 
(Davis and Poland, 1957) , and by the 
early 1950 's nearly 1 million acre-feet 
of water was being pumped from the 
aquifer system (fig. 10) . Most of the 
water was pumped from beneath the 
Corcoran Clay Member of the Tulare 
Formation. The increase in irrigated 
acreage (fig. 11) and in pumping signif- 
icantly altered the ground-water flow 
system. Percolation of irrigation water 
past crop roots greatly exceeded infil- 
tration of intermittent stream water and 
replaced the latter as the primary mech- 
anism of recharge. Discharge of water 
through wells and evapotranspiration 
from crops replaced natural evapotrans- 
piration as the primary mechanism of 
discharge. Williamson and others (1985) 
concluded that overall postdevelopment 
recharge during 1961-77 was more than 
40 times greater than the estimated 
predevelopment values for the Central 
Valley. 



1600 




1940 1945 



1950 



1960 
YEAR 



1980 1985 



FIGURE 10. - Ground-water pumpage and total available water, Westlands Water District, 1935-85. 



14 Ground-Water Flow System, San Joaquin Valley, California 



121°00' 



120°30' 



120°00' 



37°00' 



36°30 



^ 







EXPLANATION 

AREA IRRIGATED BY GROUND WATER 
1912-24 






i:::;:;:;::: : v.:J INCREASE IN AREA IRRIGATED BY 
GROUND WATER 1924-43 



: : -: : : : : : : : :-a INCREASE IN AREA IRRIGATED BY 



36=00' 



GROUND WATER 1943-55 
a AREA IRRIGATED BY SURFACE WATER 
1 AND GROUND WATER, 1912-24 

INCREASE CM AREA IRRIGATED BY SURFACE 
WATER AND GROUND WATER, 1924-43 

AREA IRRIGATED BY SURFACE WATER 
AS OF 1940 



BOUNDARY OF VALLEY DEPOSITS 




Kettleman 
City 



10 
I 



20 MILES 



10 20 KILOMETERS 

FIGURE 11. - Areal extent of areas irrigated with ground water and surface water. (Modified from Bull and Miller, 1975.) 



Pumping of ground water in the central 
part of the western valley affected the 
hydraulic head and the direction of flow 
in the system. The most pronounced 
changes occurred in the lower confined 
zone. By 1952, the potentiometric 
surface of the confined zone (fig. 12) 
was drawn down 100 to 200 feet from the 
presumed predevelopment altitude. The 
large drawdown in hydraulic head created 



a reversal in the direction of flow in 
the lower confined zone from eastward 
to westward, and also caused a signif- 
icant component of vertical flow from 
the overlying semiconfined zone. 

The changes in the altitude of the 
water table were less marked than the 
changes in hydraulic head in the confined 
zone during the early period of intensive 



Ground-Water Flow System 15 



121°00' 



120°30' 



120°00' 



37°00' 




36°30 



10 20 KILOMETERS 

FIGURE 12. - Potentiometric surface of the confined zone, 1952. (Adapted from Davis and others, 1959.) 



development. Comparison of maps of the 
altitude of the water table in 1952 
(Davis and others, 1959; fig. 13) and 
in 1908 (Mendenhall and others, 1916; 
fig. 9) indicates a lowering of the water 
table along the distal-fan margins and 
the valley trough. Water-table declines 
in these areas were probably the result 
of pumping from the Sierran sands. In 
contrast to the lowering of the water 



table along the distal-fan margins and 
the valley trough, the altitude of the 
water table seems unchanged to slightly 
elevated (Davis and Poland, 1957) along 
the western part of the area from 1906 to 
1952, except for the development of an 
oblate mound southeast of the Panoche 
Creek. It is possible, however, that 
this mound was not actually present. 
Davis and others' (1959) map probably was 



16 Ground-Water Flow System, San Joaquin Valley, California 



121 °00' 



120°30' 



120°00' 



37°00' 



Los Bancs 







'• j SAN 



— 

9 



— ^9 



% 






V 



l\ 






V -> > 









36°30 



36°00' 



EXPLANATION 



—200- 



WATER-TABLE CONTOUR - Shows 
altitude of water table. Dashed where 
approximately located. Contour 
interval, 20 feet. Datum is sea level 



BOUNDARY OF VALLEY DEPOSITS 







'Canrua *- 



$«r 



?"(„ rree k '/M^*^ ^vx\\> ( 



10 

i 



20 MILES 
_l 



- 1 

10 20 KILOMETERS 

FIGURE 13. - Water-table altitude, spring 1952. (Adapted from Davis and others, 1959.) 



not corrected for land subsidence, which 
was already substantial by the time their 
map was prepared. The apparent mound is 
in the area of greatest subsidence. 

Agricultural pumping in excess of 
recharge continued for more than a 
decade after 1952 and led to continued 
lowering of the potentiometric surface 
of the confined zone. By 1967, the 



potentiometric surface (fig. 14) had 
been lowered hundreds of feet over much 
of the western valley. The large 
quantities of ground water pumped from 
the aquifer system had several signifi- 
cant effects, including steepening of 
westward gradients in the confined zone, 
substantial increase in pumping lifts, 
and land subsidence (fig. 15) . Pumping 
lifts exceeded 800 feet over parts of 



Ground-Water Flow System 17 



121 "00' 



120*30' 



120°00' 



37°00 




10 20 KILOMETERS 

FIGURE 14. — Potenbemetrk sulfate of the confined zone, Deee«her 1%7. (Adapted from Ireland and others, 1984.) 



the area and land subsidence of more 
than 2 feet occurred throughout the 
study area, with local subsidence 
reaching as much as 28 feet by 1972. 

As a result of land subsidence, 
increased pumping lifts, and water- 
quality limitations, surface water was 
imported to the western valley in order 
to decrease pumpage from the aquifer 
system. Beginning in 1967, surface water 



imported via the California Aqueduct 
began to replace pumped water as the 
primary source of irrigation supply in 
the area south of Mendota. The avail- 
ability of surface water has led to an 
increase in the total quantity of water 
applied while the quantity of water 
removed from the system by wells has been 
decreased (fig. 10) . The marked decrease 
in pumpage has allowed a recovery in 
hydraulic head throughout the aquifer 



18 Ground-Water Flow System, San Joaquin Valley, California 



121 '00' 



120°30' 



120°00' 



37°00' 



36°30 



36°00' 




10 20 KILOMETERS 

FIGURE 15. - Land subsidence, 1926-72. (Adapted from Ireland and others, 1984.) 



system. Figures 14 and 16 show the 
potentiometric surface for the confined 
zone in 1967 and ' 1984. Comparison of 
those maps indicates that hydraulic head 
in the confined zone has risen 200 to 
300 feet from 1967 to 1984 along the 
western part of the study area in areas 
previously characterized by the largest 
drawdown. Hydraulic head in the valley 
trough has typically risen 100 feet along 
the valley trough. Overall, the rise in 



the potentiometric surface from 1967 to 
1984 has been nearly one-half the draw- 
down that occurred from predevelopment 
conditions to 1967. 

Agricultural development also has 
affected the semiconfined zone which 
overlies the Corcoran Clay Member. In- 
creased rates of recharge resulting from 
percolation of irrigation water (as com- 
pared to predevelopment recharge rates) , 



Ground-Water. Flow System 19 



120°30' 



120°00' 




10 20 KILOMETERS 

FIGURE 16. - Potentiometric surface of the confined zone, spring 1984. 



combined with the rapid post-1967 de- 
crease in pumpage, have caused a rise 
in the altitude of the water table over 
much of the western valley. Comparison 



of maps of the depth to the water table 
in 1952 (fig. 17) and 1984 (figs. 18 and 
19) indicates this marked change in the 
system. In 1984, about one-half of the 



20 Ground-Water Flow System, San Joaquin Valley, California 



37°00' 



36°30' 



^73 



121°00' 



'20°30' 



120°00' 



I. 

- SAN 

¥reser\ 




Cantua 



EXPLANATION 

—100-- LINE OF EQUAL DEPTH TO 

WATER TABLE - Interval, in feet 
is variable. Datum is land surface 

BOUNDARY OF VALLEY DEPOSITS 



<-.. .._ - ■■... 

\ 



36° 00' 



_^ ^ 



,-cttleman 
' --■■ ■- \ Clt >' 







—T 
10 



10 

_J_ 



20 MILES 



T 



10 20 KILOMETERS 

FIGURE 17. - Depth to water table, spring 1952. (Adapted from Davis and others, 1959.) 



Ground-Water Flow System 21 



12T00' 



120°30' 



120°00' 



37°00 



36°30 



36°00' 




10 20 KILOMETERS 

FIGURE 18. - Depth to water table, October 1984. 



22 Ground-Water Flow System, San Joaquin Valley, California 



37 o 00' 



36°30' 



36° 00' 




I 1 ■— I ' 

10 20 KILOMETERS 

FIGURE 19. - Wells used to map the depth to and altitude of the water table, October 1984. 



Ground-Water Flow System 23 



study area was characterized by a water 
table within 20 feet of the land surface, 
whereas in 1952 only a small percentage 
of the western valley was underlain by 
shallow ground water. Indeed, much of 
the present-day area underlain by a water 
table within 20 feet of the land surface 
was characterized by a water table at 
depths of 100 to 200 feet in 1952. The 
development and growth of areas with a 
shallow water table and resulting drain- 
age problems has been a key concern of 
agricultural interests in the area. 

Wells completed in the deepest parts of 
the semiconfined zone indicate that the 
rise in hydraulic head is larger in the 
deeper parts of the semiconfined zone 
than for the shallower parts. Figure 20 
shows hydrographs for two wells near the 
California Aqueduct. Both wells are at 
a land-surface altitude of 320 feet but 
are drilled to different depths. From 
1975 to 1984, the shallow well shows a 
rise in water level of about 20 feet and 
the deep well shows a rise of 40 feet. 
Moreover, the hydraulic head rise in the 
confined zone has been about 100 feet at 
that location during the same time 
period. These observations indicate that 
the downward head gradient is decreasing 
with time. This decrease is probably 
due to the decrease in pumpage from 
beneath the Corcoran Clay Member. 

In contrast to most of the study 
area, comparison of the depth to the 
water table in 1952 (fig. 17) and 1984 
(fig. 18) along the western margin of the 
alluvial fans indicates that the water- 
table altitude has declined along most of 
the western part of the alluvial fans. 
In 1952, the water table was generally 
within 200 feet of the land surface along 
the western margins of the alluvial fans, 
whereas, in 1984, the water table was 
typically more than 300 feet beneath the 
land surface in those areas. The increase 
in depth to the water table is especially 
pronounced beneath the fanhead of the Los 
Gatos Creek alluvial fan where in 1952, 
the water table was within 200 feet 
of the land surface over most of the 



300 



lu 280 



260 



O 
< 

u. 
0C 

05 
DC 
UJ 

h- 
< 

s 



240 



220 



200 



1974 




1976 



1982 



1984 



1978 1980 
DATE 

FIGURE 20. - Hydrographs of two wells drilled to different depths 
in the semiconfined zone. Well locations are shown on figure 21. 



fanhead. in contrast, the depth to 
the water table in 1984 was more than 
400 feet over parts of the fanhead of 
the Los Gatos Creek alluvial fan. It 
is important to note that the map of 
the depth to the water table in 1952 
was constructed, in part, by using elec- 
trical resistivity logs and that the map 
is considered approximate (Davis and 
others, 1959). Therefore, the numerical 
values of water-table decline cited in 
this study also need to be considered 
approximate. 

The decline of the water table from 
1952 to 1984 is probably a result of the 
large overdraft from the confined zone 
that occurred prior to the importation of 
surface water. The areas of water-table 
decline along the western part of the 
alluvial fans are correlative to the 
areas of greatest drawdown in the con- 
fined zone. Indeed, the greatest depth 
to the water table occurs within the fan- 
head of the Los Gatos Creek fan where the 
Corcoran Clay Member is absent over a 
large area (fig. 5); the absence of the 
Corcoran allows for enhanced hydraulic 
connection between the semiconfined and 



24 Ground-Water Flow System, San Joaquin Valley, California 



confined zones. Inspection of hydro- 
graphs in the areas of water-table 
decline indicates that water levels in 
wells completed in the semiconf ined zone 
have been rising since 1967. The overall 
decline of the water table from 1952 to 
1984 along the western margin of the 
alluvial fans indicate that although the 
water table is rising in those areas, 
those parts of the system have not yet 
recovered to the 1952 levels. 



A regional tile-drain collector system, 
which was installed in 1980-81, also has 
had significant effects on the ground- 
water flow system. The regional tile- 
drain collector system underlies about 
42,000 acres (65.6 square miles) of land 
west and southwest of Mendota (fig. 21) . 
During 1981-84, the drains collected an 
average of 6,900 acre-feet per year 
(0.15 cubic foot per year per square 
foot) . By diverting water that may have 



121-00' 



120°30' 



120°00' 



37<W 



36°30' 



36*00' 




EXPLANATION 

DRAINED AREA 

EQUIVALENT UNDRAINED AREA - 
Area not underlain by drains but of 
approximately equal size and topographic 
and geomorphic location as drained area 

GEOHYDROLOGIC SECTION 

OBSERVATION WELL AND NUMBER - 
Well used in constructing hydrographs in 
figures 20 and 22 



BOUNDARY OF VALLEY DEPOSITS 



10 

I 



20 MILES 

_J 



10 



20 KILOMETERS 



FIGURE 21. - Location of area serviced by the regional tile-drain system and an area of approximately equivalent size and 

topographic and geomorphic location. 



Ground-Water Flow System 25 



otherwise recharged the ground-water flow 
system, the drains have lowered water 
levels in the drained area from 1 to 3 
feet on a regional scale and up to 5 feet 
on a local scale. In addition, the 
drains have decreased seasonal variation 
in water levels in the drained area. 

By lowering water levels 1 to 3 feet 
in the drained area, the tile-drain 
collector system has been effective in 
decreasing the total area characterized 
by a water table within 5 feet of the 
land surface. Maps of depth to the water 
table (Westlands Water District, written 
commun., 1986) indicate that in April 
1976 about 27 square miles of the area 
later serviced by drains had a water 
table within 5 feet of the land surface. 
In April 1984, the size of the area 
underlain by a water table within 5 feet 
of the land surface had been reduced to 
4 square miles. In contrast, in an area 
of equivalent size and topographic and 
geomorphic location but not underlain by 
regional tile drains (fig. 21) , the size 
of the area underlain by a water-table 
within 5 feet of the land surface in- 
creased from 8 square miles in April 1976 
to 18 square miles in April 1984. These 
large changes in area reflect changes in 
water levels of only 1 to 3 feet. 

Figure 22 shows hydrographs of eight 
wells along a west to east line through 
an area where the Panoche Creek and 
Cantua Creek alluvial fans coalesce 
(fig. 21) . The west-to-east line is 



nearly perpendicular to the contours of 
the altitude of the water table and thus 
nearly corresponds to the lateral com- 
ponent of a flow line. The hydrographs 
illustrate the effects of the drains on 
the ground-water flow system, and also 
illustrate the local-scale variability 
of the ground-water flow system. In 
general, shallow wells in the drained 
area (more than 50) show smaller seasonal 
variation in water levels since the 
drains were installed than before and 
also show smaller seasonal variation 
than several hundred shallow wells in 
the area not underlain by drains. In 
addition, water levels in the drained 
area have been steady since the drains 
were installed, whereas wells in the 
undrained areas commonly show a rise 
in water levels. Wells 7, 8 and 9 
(fig. 22) are in the drained area and 
show smaller seasonal variation in water 
levels since the drains were installed 
than before and show constant water levels 
since 1981. Well 6 is at the edge of the 
drained area but the water levels in that 
well seem unaffected by the installation 
of the drains in 1980-81. Wells 1, 3, and 
4 are in areas not underlain by drains and 
show rising water levels with time. Wells, 
1, 3, and 4 also show seasonal variations 
in water levels of 1 to 5 feet. Well 1 is 
in close proximity to the California 
Aqueduct and does not show seasonal varia- 
tion in water levels for the period of 
record. Water-level changes in the study 
area are quite variable and many wells 
deviate from the general trends. 



26 



Ground-Water Flow System, San Joaquin Valley, California 



300 


I 
















| [ 


i I 




~\ ' 




i 








i 






^^^ 




to 

a 

m 



• 

* 

o 

♦ 

I 




I 


E DEPTH 
48 

30 

29 

20 

20 

30 - 

11 

19 A °* 










p. 


,0^~ 




280 


ELL NO ALTITUD 
1 320 

3 274 

4 255 

5 238 

6 221 

7 205 

8 192 

9 182 






n L. 


__^ 








-^ 


i 






I 

! 














I 




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I 




m_ 


9fi(1 
















P^f 


1- 
















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111 
LL 












3 vM 




Al i ►o-n 7^ 


z 












rpv^v 




r Hy^ 






UJ 

§ 240 




y ^" Vo^^/^^f 










JK^f~ 




i 




i | 




-i 


x/ 


_ _ ^ 




«^^_>*_ 


UJ 


s^x'r sht^^^ 




" 


< 




I 








































_r 


(£ 


















6 ^^V^^^^^ ^^ 


(- 
< 
















! i r i 


5 














k ^ifc^' 






i**** 




20U 






^ , A"^A^* A 








* : A 


* *^^" **. 


**ii**~ 




7JV****" 






















i 


















u< v, 


p^>o 


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I ! 






















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! 




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^>oo4x>v|xxxy 


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X><5 














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160 




















I 





1967 



1969 



1971 



1973 



1975 



1977 
DATE 



1979 



1981 



1983 



1985 



FIGURE 22. - Hydrographs of eight wells along an approximate flow line. Well locations shown on figure 21. 



Ground-Water Flow System 27 



Present-Day Configuration 
of the Water Table 

The present-day configuration of the 
altitude of the water table is shown in 
figure 23. The water table demarcates 
the top of the zone of saturation. In 
many investigations, flow is assumed to 
be horizontal and equipotential lines 
vertical. If these assumptions were 
true, then the areal distribution of the 



altitude of the water table could be taken 
as representative of the areal distri- 
bution of hydraulic head for the aquifer. 
In the western San Joaquin Valley, ver- 
tical flow is substantial, and thus the 
altitude of the water table is not repre- 
sentative of the hydraulic head at other 
depths. The map showing altitude of the 
water table can be used to determine the 
general direction of the lateral component 
of flow in the semiconfined zone. 



121°00' 



120°30' 



120°00' 



37°00 



36°30 



36°00' 




10 20 KILOMETERS 

FIGURE 23. - Water-table altitude, October 1984. 



28 Ground-Water Flow System, San Joaquin Valley, California 



One of the most notable features of 
the water table is the ground-water 
divide that more or less parallels the 
western boundary of the alluvial fans. 
The ground-water divide shifts westward 
between the fanheads of the major 
alluvial fans and shifts eastward near 
the fanheads. To the east of the ground- 
water divide, the water table lies at 
shallow depths and is a subdued replica 
of the topography. East of the divide, 
flow is eastward and northeastward, 
reflecting the general trend of the topo- 
graphy. In part of the study area, there 
is a component of flow eastward across 
the valley trough toward pumping wells 
perforated in the Sierran sands. To the 
west of the ground-water divide, the 
water table slopes steeply to the west, 
opposite in direction to the land sur- 
face. West of the divide, flow is toward 
the Coast Range and toward the fanhead 
areas. 

The existence of the ground-water 
divide is related to historical agri- 
cultural activity in the area and to the 
texture of the subsurface deposits. Com- 
parison of the present-day altitude of 
the water table (fig. 23) with the alti- 
tude in 1952 (fig. 13) indicates a lower- 
ing of the water table over at least part 
of the area to the west of the divide and 
a rise in the water table over much of 
the area to the east of the divide. The 
area of the lowered water table along the 
western boundary of the alluvial fans 
corresponds to the area of maximum draw- 
down in hydraulic head in the confined 
zone; both can be related to historic 
pumpage which occurred primarily from the 
confined zone. The rise in the water 
table to the east of the divide is pro- 
bably related to increased rates of 
recharge to the system arising from per- 
colation of irrigation water past crop 
roots. 

The location of the ground-water divide 
is partly affected by the texture of the 
subsurface sediments. The ground-water 
divide shifts eastward in the fanhead 



areas where the sediments are coarsest 
and shifts westward in the interfan areas 
where the sediments are finest grained. 
The altitude of the water table is the 
lowest, and the ground-water divide 
farthest to the east, beneath the fan- 
head of the Los Gatos Creek alluvial fan. 
The area of low altitude of the water 
table beneath the fanhead of the Los 
Gatos Creek fan corresponds to the area 
where the Corcoran Clay Member of the 
Tulare Formation is not present. The 
absence of the Corcoran results in 
better hydraulic connection between the 
semiconfined and confined zones. 

Since texture affects the specific 
yield and the permeability of the 
deposits, texture affects system response 
to hydraulic stresses. The dominant 
boundary conditions on the semiconfined 
zone are percolation of irrigation water 
past crop roots from above and the hy- 
draulic head of the confined zone from 
below (which is in turn affected by 
pumping) . Percolation of irrigation 
water past crop roots has resulted in 
accretion to the water table over a large 
area. « Because fine-grained deposits tend 
to have smaller specific yield than 
coarse-grained deposits, the water table 
accretion tends to be largest in areas of 
fine-grained deposits, such as interfan 
areas. The combined effect of pumping 
from below the Corcoran and percolation 
from above the water table has been 
development of a large downward flow 
gradient in the semiconfined zone. 
Because the fine-grained sediments tend 
to have lower permeability than the 
coarse-grained sediments, a larger 
vertical head gradient is required to 
transmit a given quantity of water 
through the fine-grained deposits than 
through the coarse-grained deposits. 
Consequently, the interfan areas have 
larger vertical head gradients than the 
fanhead areas. The larger vertical head 
gradient translates to a thicker saturated 
column in the interfan areas and hence 
shallower depth to ground water than in 
the fanhead areas. 



Ground-Water Flow System 29 



Vertical Gradients 

Under natural conditions, the ground- 
water flow system of the western San 
Joaquin Valley was characterized by 
horizontal flow over most of the area. 
Numerical simulation of the flow system 
by Williamson and others (1985) indicates 
that vertical gradients were small and 
limited to the fanhead areas (where 
there was a downward gradient) and to 
the valley trough (where there was an 
upward gradient) . Presently, the flow 
system above the Corcoran Clay Member of 
the Tulare Formation is characterized by 
a large component of vertical flow over 
most of the study area. 

Vertical gradients of hydraulic head 
were calculated at 37 locations in the 
study area. At locations with two or 
more wells, the gradient was calculated 
by dividing the difference in water 
levels by the distance between the mid- 
points of the perforated intervals. At 
locations with a single deep well, grad- 
ients were calculated only at sites where 
shallow, water levels could be interpo- 
lated to within 3 feet with a fair degree 
of confidence from the water table map 
(fig. 23). Vertical gradients during 
1984-86 ranged from 0.003 to 1.1. i n 
contrast, horizontal head gradients in 
the western valley typically ranged from 
0.001 to 0.02. 

Vertical gradients in hydraulic head 
are lower in the coarse-textured Sierran 
sands and fanhead alluvium than in the 
finer textured midfan, distal-fan, and 
flood-basin deposits. The highest gradi- 
ents occur beneath the California Aque- 
duct in areas of fine-grained deposits 
and in the flood-basin clays in areas 
where there is pumping-induced drawdown 
in the Sierran sands. Vertical gradients 
within the coarse-textured Sierran sands 
and fanhead alluvium ranged from 0.003 to 
0.07 in 1984-86. Gradients in the midfan 
areas ranged from 0.07 to 0.32 in 1984-86 
except beneath the California Aqueduct 
where gradients ranged from 0.08 to 1.0. 
Gradients in the fine-textured flood- 
basin deposits ranged from 0.10 to 1.1. 



Gradients near to one and equal to 
one beneath the aqueduct may be 
indicative of local perched conditions, 
in which saturated deposits overlie 
unsaturated deposits. The possible 
perching may be the result of pre- 
construction ponding along the canal 
right-of-way and from leakage from 
the canal. Gradients near to one and 
larger than one in the valley trough 
m the vicinity of San Joaquin and 
Tranquility may be indicative of 
perching over a larger area. Pumping 
of ground water from the Sierran 
deposits has lowered water levels in 
the Sierran sands below the altitude 
of the interface between the overlying 
flood-plain deposits and Sierran sands 
producing an unsaturated zone between 
the fine-grained flood-plain deposits 
and the Sierran sands. The low diffu- 
sivity of the clays in the flood-plain 
deposits has allowed these deposits to 
remain saturated as the water table in 
the semiconfined zone declined below the 
interface. 

Pumping from the Sierran sands was 
greater in the past than it is presently 
The present-day (1987) extent of the 
perched zone probably is smaller than it 
has been in the past and the area of 
perched conditions will continue to 
decrease with a continued decrease in 
pumping. 



Generalized Geohydrologic Section 
Through the Flow System 

A generalized geohydrologic section 
through the flow system is shown in 
figure 24. The cross section extends 
from the fanhead of the Panoche Creek 
alluvial fan to Mendota (fig. 21) . The 
section shows the generalized geology and 
distribution of hydraulic head. The 
geology of the section was interpreted 
from published maps and sections (Page, 
1986; U.S. Bureau of Reclamation, 1965). 
Direct observations of hydraulic head are 
available from several sources. Obser- 
vations of the vertical distribution of 
head in the semiconfined zone and across 



30 



Ground-Water Flow System, San Joaquin Valley, California 



WEST 



EAST 



FEET 
600 




2 KILOMETERS 



EXPLANATION 



GENERALIZED DIRECTION OF 
GROUND-WATER FLOW 



— *■ WATER TABLE * 

-200 POTENTIOMETRIC CONTOUR - Shows 

altitude at which water level would have 

stood in tightly cased wells. Dashed 

where approximately located. Contour 

interval, 25 feet above Corcoran Clay 

Member: 100 feet within Corcoran Clay 

Member. Datum is sea level 

FIGURE 24. — Generalized geohydrologic section through the flow system. Location of section shown on figure 21. 



the Corcoran Clay Member of the Tulare 
Formation are available from wells drill- 
ed to multiple depths by the U.S. Geolog- 
ical Survey at three sites: P6, P4, and 
PI (fig. 21) . The altitude of the water 
table is well documented in areas where 
the water table is within 20 feet of the 
land surface because the Westlands Water 
District maintains a network of shallow 
wells to monitor the water table in those 
areas. The distribution of hydraulic 
head beneath the Corcoran Clay Member is 
known from maps of the potentiometric 
surface of the confined zone prepared by 
the Westlands Water District and by the 
California Department of Water Resources, 
and from wells drilled by the U.S. 
Geological Survey. 

The distribution of hydraulic head 
within the semiconfined zone in regions 
without wells was inferred by interpola- 
tion between wells and based on the 



hydrogeology of the system. In particu- 
lar, the orientation of the equipotential 
lines in the area between the P6 and P4 
sites was drawn to reflect the distribu- 
tion of electrical resistivity as mapped 
by R. Bisdorf (U.S. Geological Survey, 
written commun., 1986). The equipoten- 
tial lines were contoured more vertically 
where the resistivity is indicative of 
coarse-grained deposits and were con- 
toured more horizontally where the 
resistivity is indicative of fine-grained 
deposits. The distribution of hydraulic 
head in the Corcoran reflects the known 
and interpreted values of hydraulic head 
both above and below the clay. 

The section as drawn shows a nearly 
vertical equipotential line in the 
Sierran sands. Time-series data for the 
wells at the Mendota Airport indicate 
that the vertical gradient varies season- 
ally at that site. During the late autumn 



Ground-Water Flow System 31 



and winter, the vertical head gradient is 
as low as 0.003 and in the late spring 
and summer, the head gradient is as high 
as 0.07. The increased vertical head 
gradient during the late spring and 
summer is probably attributable to nearby 
pumping. 

Generalized arrows are drawn on the 
section to indicate the general direc- 
tions of ground-water flow. The arrows 
illustrate several major aspects of the 
flow system. Ground water east of the 
ground-water divide flows downward toward 
the confined zone and eastward toward 
pumping wells in and located east of the 
valley trough. The orientation of the 
arrows east of the ground-water divide 
reflects the contrast in hydraulic pro- 
perties between the Coast Range alluvium 
and Sierran sands. The more vertically 
oriented arrow in the Coast Range alluv- 
ium is refracted toward the horizontal 
upon entering the Sierran sands. Ground 
water west of the ground-water divide 
flows toward a trough in the water table 
and downward toward the confined zone. 
The eastward-pointing arrow near the 
fanhead of the Panoche fan reflects the 
effects of the ground-water mound 
beneath the Panoche Creek (fig. 23) . 
The arrows also indicate downward flow 
across the Corcoran Clay M-ember from the 
semiconfined zone to the confined zone. 



CONCLUSIONS 

The Pleistocene Corcoran Clay Member of 
the Tulare Formation divides the ground- 
water flow system of the western San 
Joaquin Valley into an upper semiconfined 
zone and a lower confined zone. The 
deposits of the semiconfined zone can be 
divided into three hydrogeologic units: 
Coast Range alluvium, Sierran sands, and 
flood-basin deposits. The texture of the 
Coast Range alluvium varies as a function 
of position on the alluvial fans. The 
deposits are coarse textured at the heads 
of fans and along present-day stream 
channels and paleo-channels. The depos- 
its are fine textured between channels 
and at the distal-fan margins. The Coast 
Range alluvial sediments were deposited 



under arid conditions and are generally 
oxidized. The Sierran sands, in the 
valley trough, are reduced deposits of 
coarse texture. The flood-basin deposits 
are predominantly fine textured, moderate 
to densely compacted clays; the oxidation 
state of these deposits is variable. 

Agricultural development has signifi- 
cantly altered the ground-water flow sys- 
tem in the central part of the western San 
Joaquin Valley. Percolation of irrigation 
water past crop roots greatly exceeds and 
has replaced infiltration of intermittent 
streamflow as the primary mechanism of 
recharge. Pumpage of ground water from 
wells and crop evapotranspiration have 
replaced natural evapotranspiration and 
seepage to streams in the valley trough 
as the primary mechanisms of discharge. 
Historic pumpage of ground water from 
beneath the Corcoran Clay Member has 
lowered the potentiometric surface in the 
confined zone hundreds of feet over much 
of the western valley and has lowered the 
water table beneath parts of the fanheads 
of the alluvial fans. Percolation of ir- 
rigation water past crop roots has caused 
the water table to rise over a large part 
of the western valley since 1952. Surface- 
water deliveries from the California 
Aqueduct have caused a decrease in pumpage 
since 1967 and a consequent recovery in 
hydraulic head throughout the aquifer 
system. 

Increased recharge by percolation of 
irrigation water past crop roots and 
historic pumpage of ground water from 
beneath the Corcoran Clay Member have 
created a ground-water divide along the 
western margin of the valley. The divide 
is more or less parallel with the Coast 
Range but is closer to the Coast Range in 
fine-textured interfan areas and farther 
from the Coast Range in coarse-textured 
fanhead areas. The combination of perco- 
lation and pumpage also has resulted in 
development of a downward component of 
flow in the semiconfined zone. The down- 
ward component of flow is decreasing with 
time in response to reduced pumping. The 
present-day flow system is in a transient 
state and is adjusting to stresses placed 
upon it in both the past and present. 



32 Ground-Water Flow System, San Joaquin Valley, California 



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34 



Ground-Water Flow System, San Joaquin Valley, California 




P00001878